Polymers have for long been used as replacement material for other materials, such as metals. They have the advantage of being light-weight materiel, which are relative easy to shape. However, polymers do typically have lower mechanical strength compared to metals. Further, they are less heat resistant.
The need for resistant polymers led to the development of aromatic polyimides. Polyimides are polymers comprising imide bonds. Aromatic polyimides are typically synthesized by condensation of aromatic carboxylic acid dianhydride monomers, such as pyromellitic dianhydride, 4,4′-oxydiphthalic anhydride, 2,2-bis-[4-(3,4-dicarboxyphenoxy)phenyl]-propane dianhydride, 3,3′,4,4′-benzophenonetetracarboxylic acid dianhydride or 3,3′,4,4′-tetracarboxybiphenyl dianhydride, with aromatic diamine monomers, such as 4,4′-oxydianiline, 1,4-diaminobenzene, 1,3-diaminobenzene, 1,3-bis-(4-aminophenoxy)benzene, 1,3-bis-(3-aminophenoxy)benzene, methylenedianiline or 3,4′-oxydianiline.
Polyimides obtained via condensation of pyromellitic dianhydride and 4,4′-oxydianiline are among others sold under the trademarks Vespel® and Meldin®. They are materials which are lightweight and flexible, and which have good resistant to heat and chemicals.
Further, thermoset polyimides have inherent good properties, such as wear and friction properties, good electrical properties, radiation resistance, good cryogenic temperature stability and good flame retardant properties. Therefore, they are used in the electronics industry for flexible cables, as an insulating film on magnet wire and for medical tubing. Polyimide materials are also used in high or low temperature exposed applications as structural parts were the good temperature properties is a prerequisite for the function.
The need to improve the processability, while keeping the mechanical properties, of polyimides for use in airplanes and aerospace applications led to the introduction of cross-linking technologies. As the polymer chains are cross-linked, they may be shorter whilst the mechanical properties are maintained or even improved. Shorter polymer chains have the advantage of being easier to process, as the viscosity of the polymer melt or solution of the polymer is lower.
Examples of such cross-linking technologies include the bismaleimides and the nadimide-based PMR resins, which undergo cure at temperatures near 250° C. However, such thermoset polyimides will not withstand oxidative degradation on long-term exposure at temperatures above 200° C., as the cross-linking moieties have inferior thermal stability, compared to the oligoimide units.
In attempts to improve the thermal stability, thermoset polyimides containing phenylethynyl-substituted aromatic species as the reactive moieties have been developed.
U.S. Pat. No. 5,567,800 discloses phenylethynyl terminated imide oligomers (PETIs). Such oligomers may be prepared by first preparing amino terminated amic acid oligomers from dianhydride(s) and an excess of diamine(s) and subsequently end-cap the resulting amino terminated amic acid oligomers with phenylethynyl phtalic anhydride (PEPA). The amic acid oligomers are subsequently dehydrated to the corresponding imide oligomers.
Upon heating the triple bonds will react and cross-link the end-capped polyimid, thereby further improving its heat resistance and mechanical strength. As disclosed by U.S. Pat. No. 5,567,800 heating to at least 350° C. is necessary to cure the PETI.
However, for some applications the high curing temperature may be considered a problem. For instance, the properties (such as the coefficient of thermal expansion) of flexible polyimide films, having a melting temperature below 350° C., may be improved via cross-linking. However, the high temperature (above 350° C.) needed to initiate cross-linking will make the processing impossible.
If the curing temperature may be lowered, a film may be formed from a solution. During the drying step, curing may then be initiated by heating the film without melting it.
As an alternative to PEPA, also ethynyl phtalic anhydride (EPA) has been used as cross-linker in polyimides (Hergenrother, P. M., “Acetylene-terminated Imide Oligomers and Polymers Therefrom”, Polymer Preprints, Am. Chem. Soc., Vol. 21 (1), p. 81-83, 1980). Although polyimides comprising EPA may be cross-linked at a somewhat lower temperature, i.e. at about 250° C., it suffers from other drawbacks. The exchange of the phenyl ethynyl group to an ethynyl group implies that other reaction pathways than the desired curing mechanism, such as chain extension, are favored. As a consequence, EPA has not found any wide use as a replacement to PEPA as a low temperature curing end-capper. Further, the manufacture of EPA requires protective group chemistry hampering its commercial potential.
U.S. Pat. No. 6,344,523 addresses the disadvantageous of the high curing temperature discussed above and discloses that use of sulfur or organic sulfur derivatives as curing promoters may lower the curing temperature of PETI. However, the introduction of such promotors suffers from other disadvantages. In particular the curing results in chain extension rather than cross-linking as two ethynyl groups react along with one sulfur radical ultimately forming a thiophene structure.
After introducing phenylethynyl based cross-linking concept for polyimides, it has also been employed in other polymers than polyimides, such as poly(arylene ether)s (cf. J. W. Connell et al “Oligomers and Polymers Containing Phenylethynyl Groups”, Polymer Reviews, Vol. 40(2 & 3), 2000, pp. 207-230). Similar to acetylenic polyimides, also other acetylenic polymers suffers from the need to employ high temperature in order to initiate cross-linking.
In EP 2 025 698, EP 1 995 635, EP 1 669 389 and US 2004/0147710, all relating to cross-linkable aromatic fluorinated pre-polymers, it is stated that amines such as aniline, i.e. a nuclephilic primary, i.e. non-tertiary, amine, triethylamine, aminophenyltrialkoxysilane and aminopropyltrialkoxysilane and organic metal compound containing molybdenum, nickel or the like, may be used as catalysts for pre-polymer containing ethynyl groups as the cross-linkable functional groups. However, said effect is not supported by experimental data.
Thus, there is need within the art for ways of lowering the temperature at which cross-linking of acetylenic oligomers or polymers are initiated.